Seite - 56 - in The Origin of Species

56 ORIGIN OF SPECIES sert that the limit has been attained in any one case ; for al- most all our animals and plants have been greatly improved in many w^ays within a recent period ; and this implies variation. It vi^ould be equally rash to assert that characters now in- creased to their usual limit, could not, after remaining fixed for many centuries, again vary under new conditions of life. No doubt, as Mr. Wallace has remarked with much truth, a limit will be at last reached. For instance, there must be a limit to the fleetness of any terrestrial animal, as this will be determined by the friction to be overcome, the weight of body to be carried, and the power of contraction in the mus- cular fibres. But what concerns us is that the domestic vari- eties of the same species differ from each other in almost every character, which man has attended to and selected, more than do the distinct species of the same genera. Isi- dore Geoffroy St. Hilaire has proved this in regard to size, and so it is with colour and probably with the length of hair. With respect to fleetness, which depends on many bodily char- acters, Eclipse was far fleeter, and a dray-horse is incom- parably stronger than any two natural species belonging to the same genus. So with plants, the seeds of the different varieties of the bean or maize probably differ more in size, than do the seeds of the distinct species in any one genus in the same two families. The same remark holds good in re- gard to the fruit of the several varieties of the plum, and still more strongly with the melon, as well as in many other anal- ogous cases. To sum up on the origin of our domestic races of animals and plants. Changed conditions of life are of the highest importance in causing variability, both by acting directly on the organisation, and indirectly by affecting the reproductive system. It is not probable that variability is an inherent and necessary contingent, under all circumstances. The greater or less force of inheritance and reversion determine whether variations shall endure. Variability is governed by many unknown laws, of which correlated growth is probably the most important. Something, but how much we do not know, may be attributed to the definite action of the conditions of life. Some, perhaps a great, effect may be attributed to the increased use or disuse of parts. The final result is thus